5-Fluoroorotic Acid (5-FOA) is used in yeast molecular genetics to detect expression of the URA3 gene, which encodes orotine-5'-monophosphate (OMP) decarboxylase. 5-FOA is toxic to yeast cells that can synthesize the enzyme orotidine-5’-phosphate decarboxylase and are therefore unable to grow in 5-FOA-containing media.